1. Field of the Invention
The present invention relates generally to powder densifying and is more particularly concerned with a method for densifying sub-micron powder materials of low apparent density.
2. Description of the Prior Art
Many materials of commerce are, as originally produced, in the form of light, fluffy and voluminous powders of low apparent densities. Exemplary of these are the so-called "fumed" metal and metalloid oxides produced by high temperature vapor phase oxidation or hydrolysis of compounds of the corresponding metals or metalloids. So too are the carbon blacks produced by any of the well-known furnace, thermal, plasma or channel processes. Such powder materials are normally first collected in the form of sub-micron powders having apparent densities of less than about 25 kg/m.sup.3. In this very light and voluminous state these powder materials are expensive to package and ship since they occupy substantial volume per unit weight thereof. Too, the ultimate consumer of such powder materials often experiences difficulties in their handling and/or in their compounding into finished product formulations since, if not first densified, powder materials of low apparent densities tend to be dusty and tend to bridge hopper outlets, plug orifices, form unwanted deposits and cause other innumerable difficulties in their conveyance, metering and handling.
In an effort to resolve such difficulties it is common practice to subject sub-micron powder materials of low apparent densities to one or more various densifying treatments prior to their shipment or use. Of course, in the densifying of such materials, due precautions need be taken to prevent deleterious alteration of their desirable properties. For instance, it is usually important that densifying of such materials as fumed silicas and carbon blacks be undertaken in such manner that the dispersibility of the diversified powder product in the intended end item composition or formulation is not excessively deleteriously affected.
Carbon blacks find a major use as reinforcing agents in natural and synthetic rubber formulations. In this application, the end item rubber formulation is usually produced by mastication of the solid ingredients of the formulation, including the carbon black, into the elastomeric stock. The shear or mixing forces generated under typical rubber mastication conditions, such as is accomplished in roll mills or internal mixing equipments of the Banbury type, is generally of a sufficiently high order as to allow the carbon black manufacturer to supply the carbon black in a densified agglomerated or pelleted form without substantial degradation of the reinforcing properties of the carbon black. Carbon blacks also find extensive utility as black pigments in paints, enamels, lacquers and thermoplastics. In these applications, of course, the dispersion character of the carbon black is often of major importance. Failure to produce good and uniform dispersions of carbon blacks in these end items under conventional mixing conditions can seriously and adversely affect the desired properties thereof. Moreover, since the relatively large shear forces generated during the course of normal rubber compounding operations are generally not attainable in the preparation of liquid or thermoplastic carbon black dispersions, it is not normally possible in these applications to counter poor dispersion characteristics of a particular carbon black pellet batch simply by increasing mixing time or energy.
Two types of apparatus are conventionally employed in the preparation of pelleted carbon blacks. One type is basically a rotating drum having an inlet end and a discharge end. The "fluffy" carbon black powder, which may or may not be prewetted with water or other pelletizing agents, is charged into the inlet end of the drum and is caused to tumble about therein, thereby to coalesce the minute particles thereof into larger rounded agglomerates or pellets. The formed pellets, if wetted with water, are finished by drying thereof. Exemplary of such drum pelletizing apparatus is, for instance, that disclosed in U.S. Pat. No. 2,812,541, G. J. Webster et al., Nov. 12, 1957. Another type of apparatus for densifying and pelletizing carbon blacks comprises a static cylindrical enclosure, oriented slightly off-the-horizontal, which enclosure is equipped with a coaxial rotating shaft therethrough to which there are fixed a multiplicity of radially oriented pins. The lengths of said pins are such that the free ends thereof are located substantially adjacent the walls of the enclosure. The "fluffy" carbon black powder is charged into the higher end of the enclosure and, by rotation of the agitator shaft, the radial pins are continuously driven through the bed of carbon black, thereby densifying and coalescing same into pelleted form and, in most instances, aiding to drive the mass of carbon black towards the lower discharge end of the enclosure. As with the drum pelletizing apparatus mentioned previously, various liquid pelletizing agents can be, if desired, added to the agitating bed of black within the pin pelletizer apparatus and/or the black can be prewetted prior to its introduction into the cylindrical enclosure thereof. Pin pelletizers of this type are disclosed in such patent literature as: U.S. Pat. No. 3,390,424, R. J. Fortune, July 2, 1968; U.S. Pat. No. 3,891,366, Wilson H. Rushford, June 24, 1975 and U.S. Pat. No. 4,136,975, Glenn J. Forseth, Jan. 30, 1979. The resulting densified pellets from either of the foregoing types of apparatuses, while usually adequate for rubber reinforcement purposes, are nevertheless often possessed of some detrimental characteristics, certain of which can seriously affect their performance as pigments for liquid or thermoplastic compositions. For instance, it is often found that pelleted carbon black powders of the prior art can be of non-uniform density, either in terms of pellet-to-pellet densities or in terms of intra-pellet densities. As to this last, it is often found that the density of the exterior surfaces of carbon black pellets produced in either of the foregoing types of apparatus is substantially greater than that of the interiors or cores thereof. Too, pelletizing or densifying operations of the foregoing types are often difficult to control in view of the fact that pelleted product quality and uniformity is usually dependent upon maintenance of an accurate and continuous flow of the fluffy carbon black powder feed material into the inlet ends of the pelletizer apparatuses. As mentioned previously, it is normally difficult to control the flow of such light sub-micron powders with the degree of accuracy necessary to achieve good densified product uniformity.
Fumed sub-micron silicas find extensive commercial utility as reinforcing agents for polymers, particularly silicon rubber and as thickening or thixotropic agents for various liquids, particularly polyester gel coat resins and hydrocarbon oils. As with the fluffy carbon blacks, fumed silicas are also generally found to be dusty materials which are susceptible to electrostatic charge build-up and are difficult to handle, meter and convey. Accordingly, it is conventional to densify fumed silicas by vibratory treatment thereof in a holding bin preparatory to packaging for shipment. Somewhat further densification can be achieved by vacuum bagging. Where the fumed silica is intended to be employed as a silicone rubber reinforcing agent it can be subjected to a more rigorous densifying treatment. Said treatment, and the apparatus employed therefor, is disclosed in detail in the following patent literature, all of which is in the name of Helmut Reinhardt et al.; U.S. Pat. No. 3,838,785, June 12, 1973; U.S. Pat. No. 3,742,566, July 3, 1973; U.S. Pat. No. 3,762,851, Oct. 2, 1973 and U.S. Pat. No. 3,860,682, Jan. 14, 1975. Basically, the apparatus employed is a chamber housing a pair of rolls maintained in axial parallelism and which rolls are in a fixed, spaced apart association such as to define a narrow "nip" or gap therebetween. At least one of the rolls is composed of a gas-permeable material and the interior of the roll is placed in communication with a vacuum source. The fumed silica powder material is fed to the chamber and the rolls are operated in a counter-rotating manner, thereby to capture the fumed silica powder in the nip therebetween and to express air therefrom. Meanwhile, a vacuum is developed within those roll(s) comprising the porous gas-permeable surface(s), thereby continuously withdrawing air from the interstices of the silica particles as they are compressed in the nip of the rolls. The densified silica product of this type of apparatus, and the operations thereof, are also subject to certain deficiencies. Firstly, the densified silica product tends to be non-uniform in density across its cross-section, the greatest density being achieved at the surface(s) of said product adjacent the vacuum roll(s) and the least density tending to occur at the point farthest removed from the vacuum roll(s). Secondly, while in utilizing such apparatuses it is said to be possible to achieve densification to a level of about three or four times that of the original apparent density of the fumed silica powder feed, even greater levels of densification, consistent with maintenance of good silicone rubber reinforcement properties, would be desirable. Finally, the operations of such apparatus are relatively expensive, particularly at densifying rates attractive for commercial operations. This is so because the vacuum equipments needed to handle the large rates of gas removal required from the fluffy fumed silica powder feed represent a relatively large capital expense and because such vacuum equipments normally require considerable maintenance and upkeep.
In accordance with the present invention, however, the above and other problems attendant the densification of voluminous sub-micron powder materials of low apparent density have been vastly ameliorated or substantially eliminated.